2.1. Fungal Pathogens
Fungi as a group cause many plant diseases. For purposes of discussion the fungi can be classified as belonging to one of three major taxonomic classes: Basidiomycetes, Phycomycetes, or Ascomycetes.
Basidiomycetes
Members of this class are identified by the presence of a sexual-spore forming structure known as a basidium. Pathogenic forms include smuts, rusts and fleshy species such as mushrooms. Examples include wheat rust, white pine blister, cedar-apple rust, and smuts causing disease in corn, oats, barley, onions and wheat.
Phycomycetes
Members of this class are considered to be more primitive than members of either the Ascomycetes or Basidiomycetes, their distinguishing morphological feature being the absence of mycelial crosswalls. Examples of disease caused by members of the class include the downy mildews of grape and other hosts, root rot and late blight of potato and tomato.
Ascomycetes
Members of this class possess a specialized reproductive structure (an ascus) in which meiosis and sexual spore formation take place. Examples of the more common plant diseases in which Ascomycetes have been identified as the etiologic agent include: powdery mildews on cereals, fruits and many other crops; Dutch elm disease; ergot of grains; peach and plum brown rot; black spot of roses as well as apple scab.